Mountaingoat, I have been taking my 2yr 3month old to swimming lessons since he was four months.
He can now jump in, go underwater (we put them underwater from the first lesson), swim a stroke or two, and is very happy and confident in the water. He will be swimming unaided by the time he's three. He also loves it and will play happily in the pool for as long as I let him.
I wanted him to learn asap so he never develops a fear of water. It is much, much harder to teach a four year old who's never been swimming before. I also wanted him to swim so he can be safe. And having experienced school swimming lessons, I know that if a child doesn't already know how to swim, they are generally a chaotic waste of time.
But it's up to you. I enjoy going swimming with him - it's one of our fun things we do together. I would never tell anyone else they were a bad mother for not doing the swimming thing. Like I say, I'm lucky. I can afford a nice health club.
Public swimming provision in this country is absolutely shit and is getting worse - pools are being closed all the time by money-grabbing councils and many of those that are left are being deliberately run down so they can sell them off to developers. And then the government preaches to us about obesity and wonders why we don't like taking our kids to far-away, shitty, smelly, filthy pools. And then spends £12 billiion on the fecking Olympics. Grrrr. Sorry, rant over.